Mechanical forces—pushes, pulls, tensions, compressions—are important regulators of cell development and behavior. Tensegrity provides the structure that determines how these physical forces are distributed inside a cell or tissue, and how and where they exert their influence.
In the human body, most cells are constantly subjected to mechanical forces, such as tension or compression. Application of mechanical strain to cells in culture simulates the in vivo environment, causing dramatic morphologic changes and biomechanical responses in the cells. There are both long and short term changes that occur when cells are mechanically loaded in culture, such as alterations in the rate and amount of DNA or RNA synthesis or degradation, protein expression and secretion, the rate of cell division and alignment, changes in energy metabolism, changes in rates of macromolecular synthesis or degradation, and other changes in biochemistry and bioenergetics.
Every cell has an internal scaffolding, or cytoskeleton, a lattice formed from molecular “struts and wires”. The “wires” are a crisscrossing network of fine cables, known as microfilaments, that stretch from the cell membrane to the nucleus, exerting an inward pull. Opposing the pull are microtubules, the thicker compression-bearing “struts” of the cytoskeleton, and specialized receptor molecules on the cell's outer membrane that anchor the cell to the extracellular matrix, the fibrous substance that holds groups of cells together. This balance of forces is the hallmark of tensegrity.
Tissues are built from groups of cells, like eggs sitting on the “egg carton” of the extracellular matrix. The receptor molecules anchoring cells to the matrix, known as integrins, connect the cells to the wider world. Mechanical force on a tissue is felt first by integrins at these anchoring points, and then is carried by the cytoskeleton to regions deep inside each cell. Inside the cell, the force might vibrate or change the shape of a protein molecule, triggering a biochemical reaction, or tug on a chromosome in the nucleus, activating a gene.
Cells also can be said to have “tone,” just like muscles, because of the constant pull of the cytoskeletal filaments. Much like a stretched violin string produces different sounds when force is applied at different points along its length, the cell processes chemical signals differently depending on how much it is distorted.
A growth factor will have different effects depending on how much the cell is stretched. Cells that are stretched and flattened, like those in the surfaces of wounds, tend to grow and multiply, whereas rounded cells, cramped by overly crowded conditions, switch on a “suicide” program and die. In contrast, cells that are neither stretched nor retracted carry on with their intended functions.
Another tenet of cellular tensegrity is that physical location matters. When regulatory molecules float around loose inside the cell, their activities are little affected by mechanical forces that act on the cell as a whole. But when they're attached to the cytoskeleton, they become part of the larger network, and are in a position to influence cellular decision-making. Many regulatory and signaling molecules are anchored on the cytoskeleton at the cell's surface membrane, in spots known as adhesion sites, where integrins cluster. These prime locations are key signal-processing centers, like nodes on a computer network, where neighboring molecules can receive mechanical information from the outside world and exchange signals.
Thus, assessing the full effects of drugs, drug delivery vehicles, nanodiagnostics or therapies or environmental stressors, such as particles, gases, and toxins, in a physiological environment requires not only a study of the cell-cell and cell-chemical interactions, but also a study of how these interactions are affected by physiological mechanical forces in both healthy tissues and tissues affected with diseases.
Methods of altering the mechanical environment or response of cells in culture have included wounding cells by scraping a monolayer, applying magnetic or electric fields, or by applying static or cyclic tension or compression with a screw device, hydraulic pressure, or weights directly to the cultured cells. Shear stress has also been induced by subjecting the cells to fluid flow. However, few of these procedures have allowed for quantitation of the applied strains or provided regulation to achieve a broad reproducible range of cyclic deformations within a culture microenvironment that maintains physiologically relevant tissue-tissue interactions.
Living organs are three-dimensional vascularized structures composed of two or more closely apposed tissues that function collectively and transport materials, cells and information across tissue-tissue interfaces in the presence of dynamic mechanical forces, such as fluid shear and mechanical strain. Creation of microdevices containing living cells that recreate these physiological tissue-tissue interfaces and permit fluid flow and dynamic mechanical distortion would have great value for study of complex organ functions, e.g., immune cell trafficking, nutrient absorption, infection, oxygen and carbon dioxide exchange, etc., and for drug screening, toxicology, diagnostics and therapeutics.
The alveolar-capillary unit plays a vital role in the maintenance of normal physiological function of the lung as well as in the pathogenesis and progression of various pulmonary diseases. Because of the complex architecture of the lung, the small size of lung alveoli and their surrounding microvessels, and the dynamic mechanical motions of this organ, it is difficult to study this structure at the microscale.
The lung has an anatomically unique structure having a hierarchical branching network of conducting tubes that enable convective gas transport to and from the microscopic alveolar compartments where gas exchange occurs. The alveolus is the most important functional unit of the lung for normal respiration, and it is most clinically relevant in that it is the blood-gas barrier or interface, as well as the site where surfactants act to permit air entry and where immune cells, pathogens and fluids accumulate in patients with acute respiratory distress syndrome (ARDS) or infections, such as pneumonia.
The blood-gas barrier or tissue-tissue interface between the pulmonary capillaries and the alveolar lumen is composed of a single layer of alveolar epithelium closely juxtaposed to a single layer of capillary endothelium separated by a thin extracellular matrix (ECM), which forms through cellular and molecular self-assembly in the embryo. Virtually all analysis of the function of the alveolar-capillary unit has been carried out in whole animal studies because it has not been possible to regenerate this organ-level structure in vitro.
A major challenge lies in the lack of experimental tools that can promote assembly of multi-cellular and multi-tissue organ-like structures that exhibit the key structural organization, physiological functions, and physiological or pathological mechanical activity of the lung alveolar-capillary unit, which normally undergoes repeated expansion and contraction during each respiratory cycle. This limitation could be overcome if it were possible to regenerate this organ-level structure and recreate its physiological mechanical microenvironment in vitro. However, this has not been fully accomplished.
What is needed is a organ mimic device capable of being used in vitro or in vivo which performs tissue-tissue related functions and which also allows cells to naturally organize in the device in response to not only chemical but also mechanical forces and allows the study of cell behavior through a membrane which mimics tissue-tissue physiology.